Generally, a chemical battery is a battery composed of a positive electrode, a negative electrode, and an electrolyte to thus generate electrical energy using a chemical reaction, and is classified into a primary battery, which is disposable, and a secondary battery, which is chargeable and dischargeable and thus is repeatedly usable. The use of the secondary battery is gradually increasing thanks to the advantage of its chargeable and dischargeable property.
Among secondary batteries, a lithium secondary battery has high energy density per unit weight, and thus is widely used as a power source in electronic communication devices and for high-power hybrid vehicles.
The lithium secondary battery includes an electrode group, composed of a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and a positive electrode tab and a negative electrode tab respectively connected to the positive electrode and the negative electrode and having a rectangular shape with a predetermined width.
The positive electrode and the negative electrode are formed by applying an active material on a collector, and the positive electrode tab and the negative electrode tab are connected to respective portions of the positive electrode and the negative electrode on which the active material is not applied. On the positive electrode and the negative electrode, the portion on which the active material is applied and the portion on which no active material is applied are formed together, thus forming an active material border therebetween.
The active material is applied through a coating process, and is typically applied so that it is thicker on the active material border. Thus, there is a problem in which the entire electrode warps due to the application of the active material.
In order to prevent this problem, an electrode-pressing process may be conducted. In this case, however, wrinkles may be generated near the active material border of the electrode. The generation of wrinkles may be relatively severe on an electrode composed of a material having high elongation.
When the electrode warps or wrinkles, in the electrode group in which a plurality of electrodes is stacked, the electrodes are not uniformly brought into close contact with each other but have gaps therebetween, undesirably incurring non-uniform contact of the electrodes.
Thereby, the entire electrode group may be distorted, and the battery reaction may be non-uniformly generated. For this reason, the lifetime of the lithium secondary battery may be decreased and the electrical properties thereof may be deteriorated.
As acceleration of non-uniformity is caused by the areas of the electrodes at which the electrodes are not uniformly brought into close contact with each other, lithium may be deposited, negatively affecting the safety of the lithium secondary battery.